Mona lives in a ruined castle by the sea. She is a princess but lives a life of misery, mistreated by an odious pair of rats. But one day an adorable little unicorn U appears and declares that she is there to help her through her life and to assure her happiness. The two become inseparable. One day U brings home a charming and funny Lizard, Lazare. Scandalized by this intrusion to their home, the rats demand the newcomer leaves. In the surrounding woods they meet Lazare's family, the Wewes, a family of musicians living in a tree. Here life couldn't be more different than in the castle, full of pleasure, music and merrymaking. Mona discovers two new things: the wonder of music and a blossoming attraction to the young guitarist Kuika. However U suddenly becomes aware of a worrying dilemma. What will happen to her if Mona finds the happiness which is the sole reason for her existence...?